Observational Correlation Between Magnetic Field, Angular Momentum, and Fragmentation in the Envelopes of Class 0 Protostars?
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Astronomy & Astrophysics manuscript no. Galametz2020b © ESO 2021 January 28, 2021 Observational correlation between magnetic field, angular momentum, and fragmentation in the envelopes of Class 0 protostars? Maud Galametz1, Anaelle¨ Maury1;2, Josep M. Girart3;4, Ramprasad Rao2, Qizhou Zhang2, Mathilde Gaudel5, Valeska Valdivia1, Patrick Hennebelle1, Victoria Cabedo-Soto1, Eric Keto2, Shih-Ping Lai6 1 Astrophysics department, CEA/DRF/IRFU/DAp, Universite´ Paris Saclay, UMR AIM, F-91191 Gif-sur-Yvette, France, e-mail: [email protected] 2 Center for Astrophysics j Harvard & Smithsonian, 60 Garden street, Cambridge, MA 02138, USA 3 Institut de Ciencies` de l’Espai (ICE, CSIC), Can Magrans, S/N, E-08193 Cerdanyola del Valles,` Catalonia, Spain 4 Institut d’Estudis Espacials de de Catalunya (IEEC), E-08034 Barcelona, Catalonia, Spain 5 LERMA, Observatoire de Paris, 61 av. de l’Observatoire F-75014 Paris, France 6 Institute of Astronomy and Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan Preprint online version: January 28, 2021 ABSTRACT Aims. Our main goal in this analysis is to assess the potential role of magnetic fields in regulating the envelope rotation, the formation of disks, and the fragmentation of Class 0 protostars in multiple systems. Methods. We used the Submillimeter Array to carry out observations of the dust polarized emission at 0.87 mm in the envelopes of a large sample of 20 Class 0 protostars. We estimated the mean magnetic field orientation over the central 1000 au envelope scales to characterize the orientation of the main component of the organized magnetic field at the envelope scales in these embedded protostars. This direction was compared to that of the protostellar outflow in order to study the relation of their misalignment and the kinematics of the circumstellar gas. The latter is traced through the velocity gradient that is observed in the molecular line emission + (mainly N2H ) of the gas at intermediate envelope scales. Results. We discover that the misalignment of the magnetic field orientation is strongly related to the outflow and the amount of angular momentum observed at similar scales in the protostellar envelope. This reveals a potential link between the kinetic and the magnetic energy at envelope scales. The relation could be driven by favored B-misalignments in more dynamical envelopes or by a dependence of the envelope dynamics on the initial large-scale B configuration. By comparing the trend with the presence of fragmentation, we observe that single sources are mostly associated with conditions of low angular momentum in the inner envelope and good alignment of the magnetic field with protostellar outflows at intermediate scales. Our results suggest that the properties of the magnetic field in protostellar envelopes are tightly related to the rotating infalling gas that is directly involved in the formation of stars and disks: we find that it may not only affect the fragmentation of protostellar cores into multiple stellar systems, but also sets the conditions that establish the pristine properties of planet-forming disks. Key words. Stars: formation, protostars, low-mass, circumstellar matter – ISM: magnetic fields, kinematics and dynamics – Submillimeter: ISM – Instrumentation: interferometers, polarimeters – Methods: observational 1. Introduction the efficiency of the star formation process, the global properties of stars in our Galaxy, and the setting of the conditions that allow A majority of stars in our Galaxy are found in multiple stellar disks and planets to form around them. systems, and a significant fraction of solar-type stars will host planetary systems (Ducheneˆ & Kraus 2013; Hsu et al. 2019). Magnetic fields (hereafter B) are ubiquitous in the Universe Most of the final stellar mass is collected during a short but vig- (Vallee´ 2004) and have been observed to permeate the inter- orous accretion phase. During this so-called protostellar phase, stellar material deep down into star-forming cores and proto- arXiv:2010.12466v2 [astro-ph.GA] 27 Jan 2021 the star forms at the center of an infalling rotating core, concomi- stellar environments (Girart et al. 2006; Hull & Zhang 2019). tantly with a surrounding disk of gas in circular orbits around the From a theoretical point of view, the presence of B in star- star: while the star will inherit the majority of the accreted mass, forming cores has been shown to significantly alter the dynamics most of the angular momentum contained in the protostellar en- of the gas participating in the building of stars during the accre- velope is expected to be expelled or stored in the protostellar tion phase, and it affects the resulting properties of these stars disk. This evolution will eventually lead to protoplanetary sys- and associated circumstellar disks (Terebey et al. 1984; Wurster tems (Zhao et al. 2020). Class 0 objects are the youngest accret- & Li 2018; Hennebelle & Inutsuka 2019). The mechanism for ing protostars and are surrounded by a dense envelope that is evacuating angular momentum from the infalling gas through accreted onto the central protostellar embryo during a short (t < magnetic torques applied by Alfven´ waves is called magnetic 5×104 yr) accretion phase (Andre´ et al. 2000; Evans et al. 2009). braking. The initial global collapse, driven by gravity, drags the Characterizing the dynamics of the gas and the physical pro- field lines, which leads to an hourglass morphology of the field cesses of these youngest protostars is crucial for understanding lines and amplifies the magnetic intensity. The magnetic braking 1 M. Galametz et al: Polarized-dust emission in solar-type Class 0 protostars seems to be particularly enhanced by the pinching of field lines, ported in the literature. A potential link between the envelope which lengthens the magnetic lever arms and efficiently trans- dynamics and the B orientation might be an additional signa- ports the angular momentum from the inner envelope toward ture that B has a strong effect on the collapse and fragmentation, the outer one (Galli et al. 2006). As less angular momentum is as has also been suggested by the analysis of massive cores by transported toward the forming star, only small protoplanetary Zhang et al. (2014). disks form while the star grows (Allen et al. 2003; Hennebelle We complement the Galametz et al. (2018) observations with & Fromang 2008; Masson et al. 2016; Hirano & Machida 2019). eight additional Class 0 envelopes observed with the SMA at The importance of the misalignment between magnetic field 0.87mm at comparable scales. The full SMA B measurements and rotation axis has been stressed by several authors (Ciardi are combined with gas kinematics information obtained ho- + & Hennebelle 2010; Joos et al. 2012; Gray et al. 2018). They mogeneously from N2H observations of velocity gradients in found that in ideal magnetohydrodynamic (MHD) calculations, the envelopes, either from the Continuum And Lines in Young only small disks form or even no disk forms in the aligned con- ProtoStellar Objects survey (CALYPSO; Maury et al. 2019; figurations when the field is strong enough; and it is compara- Gaudel et al. 2020, seven sources) or from observations pub- tively far easier to form a disk in the misaligned case. Numerical lished in the literature that we reprocessed when required (12 simulations taking into account non-ideal MHD effects (such sources, see Table 4). Our goal is to observationally test the the- as ambipolar diffusion or Hall effect) were able to overcome oretical predictions of the conditions required for magnetic brak- the magnetic braking catastrophe, leading to the formation of ing affecting the collapse and assess the potential role of B in disks similar to those observed (Hennebelle et al. 2016; Zhao regulating the matter infall and envelope rotation, the formation et al. 2018; Wurster & Bate 2019). Studies by Hennebelle et al. of disks, and the fragmentation into multiple systems. (2020) or Wurster & Lewis (2020) appear to also predict that the misalignment of the magnetic field with the envelope rotation axis directly affects the protostellar disk formation, for instance, leading to the formation of larger planet-forming disks in the 2. Observations misaligned cases that were investigated compared to the smaller disks that are observed in aligned cases. This is particularly clear 2.1. Sample description when the field intensity is such that the mass-to-flux ratio is on Galametz et al. (2018) presented 12 low-mass Class 0 protostars the order of 10. Another key prediction of magnetized models is observed in polarization at 345 GHz with the SMA. We comple- that a strong, organized B partly alters the ability of the envelope ment this first subsample with 8 additional low-mass protostars. to fragment. This suggests that B is one of the regulating agents The 20 sources cover a wide range of protostellar properties: iso- that drive the birth of the multiple stellar systems we commonly lated, binary, triple, or quadruple systems form in cores whose observe in the Galaxy (Hennebelle & Teyssier 2008). masses range from 0.2 to 12M . Details of the full sample are The effect of the various characteristics of the B field (ori- provided in Table 1. entation with the collapse direction and strength), however, is poorly quantified observationally speaking. Only a few studies have attempted to test the predicted relation that links the B- 2.2. SMA dust polarization observations field orientation in protostellar cores to the magnitude of the an- gular momentum of the gas responsible for disk properties and The observations (taken in both compact and sub-compact con- the formation of multiple stellar systems (see, e.g., the works figurations), data reduction and polarization maps of the first 12 of Chapman et al. 2013; Hull et al. 2013; Zhang et al. 2014). low-mass Class 0 protostars observed are presented in Galametz Because it is difficult to trace B in these small embedded as- et al.